Relays are used in telecommunications networks to not only forward signals between mobile terminals and the core network, but to add power to the signal. An equivalent term for a relay is a repeater. These are employed in various types of telecommunication systems, including the Long Term Evolution (LTE)/System Architecture Evolution (SAE) currently being developed by the “3rd Generation Partnership Project” (3GPP). In this regard, Long Term Evolution (LTE) is an advanced version of UMTS that uses E-UTRA (Evolved Universal Terrestrial Radio Access), and which employs OFDMA (Orthogonal Frequency Division Multiple Access) on the downlink and SC-FDMA (Single Carrier Frequency Division Multiple Access) on the uplink.
The Radio Access Network component of the LTE/SAE is called the E-UTRAN, which comprises eNode Bs (eNBs). The eNBs provide both user plane and control plane (RRC) protocol terminations towards the mobile terminals (UEs) in the network. The eNBs are interconnected with each other by means of the X2 interface, and are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (S-GW) by means of the S1-U. These are the channels over which communications are made in the network.
According to 3GPP TS 36.300, there is a functional split between the different elements of the LTE network, which is illustrated in FIG. 1. In particular, it is to be noted that eNBs control the dynamic allocation of resources (i.e. scheduling) to the UE in both the uplink and downlink.
Transmissions over such wireless uplink and downlink channels are subject to errors, for example due to receiver noise and unpredictable interference variations. Therefore, virtually all wireless communications systems employ some form of Forward Error Correction (FEC). The basic principle of forward error-correcting coding is to introduce redundancy in the transmitted signal. This is achieved by adding parity bits to the information bits prior to transmission (alternatively, the transmission could consists of parity bits alone, depending on the coding scheme used). In a further variation the parity bits may be “punctured” by removing some of the parity bits after encoding with an error correction code. The parity bits are computed from the information bits using a method given by the coding structure used. Thus, the number of bits transmitted over the channel is larger then the number of original information bits and a certain amount of redundancy has been introduced in the transmitted signal.
Another approach to handle transmissions errors is to use Automatic Repeat Request (ARQ). In an ARQ scheme, the receiver uses an error-detecting code, typically a Cyclic Redundancy Check (CRC), to detect if the received packet is in error or not. If no error is detected in the received data packet, the received data is declared error-free and the transmitter is notified by sending a positive acknowledgment (ACK). On the other hand, if an error is detected, the receiver discards the received data and notifies the transmitter via a return channel by sending a negative acknowledgment (NAK). In response to a NAK, the transmitter retransmits the same information.
Hybrid ARQ (HARQ) is a combination of forward error-correcting coding and ARQ. HARQ uses forward error correcting codes to correct a subset of all errors and relies on error detection to detect uncorrectable errors. Erroneously received packets are discarded and the receiver requests retransmissions of corrupted packets.
Whilst these techniques do improve the transmission efficiency, there is still room for improvement. This is particularly the case in modern mobile communications networks, where users are demanding higher data rates. At present, typically only those mobile terminals that are in close proximity to a base station (eNB) can achieve a high data rate, as interference affects the data rate as the distance between the base station and the user terminal increases.
The use of relay nodes has been proposed to distribute the data rate more evenly in a cell served by a particular base station. This approach uses one or more relays for a single transmission. Whilst such a relay system can greatly increase the data throughput, an inherent problem with relay nodes (RN) is that in most situations, a given node cannot simultaneously transmit and receive at the same frequency band, due to the transmitting and receiving antennas not being well separated. Therefore each RN is not able to listen while transmitting, and vice versa, which introduces restrictions on their operation.
A further problem for LTE networks is that since LTE needs to be compatible with both LTE compatible terminals and legacy terminals (such as Rel-8 terminals to which relays are transparent), there is a challenge to integrate relays in such a network environment without unduly increasing network signalling.
It is therefore desirable to devise an improved mechanism for incorporating relay nodes into mobile networks.